Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
suyuti:9445a –
Request/Fix translation

  

السيوطي:٩٤٤٥a –

"أَيُّمَا رَجُلٍ مُسْلِمٍ أكْفَرَ رَجُلًا مُسْلِمًا فَإِنْ كَانَ كَافِرًا، وَإلَّا كَانَ هُوَ الكَافِر".  

[د] أبو داود عن ابن عمرو

See similar narrations below:

Collected by Abū Dāwūd
abudawud:4687 – ʿUthmān b. Abū Shaybah > Jarīr > Fuḍayl b. Ghazwān > Nāfiʿ > Ibn ʿUmar

If any believing man calls another believing man an unbeliever, if he is actually an infidel, it is all right ; if not, he will become an infidel.  

أبو داود:٤٦٨٧ – حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا جَرِيرٌ عَنْ فُضَيْلِ بْنِ غَزْوَانَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ قَالَ

قَالَ رَسُولُ اللَّهِ ﷺ أَيُّمَا رَجُلٍ مُسْلِمٍ أَكْفَرَ رَجُلاً مُسْلِمًا فَإِنْ كَانَ كَافِرًا وَإِلاَّ كَانَ هُوَ الْكَافِرَ